CAISSON DISEASE. In order to exclude the water, the air pressure within a caisson used for subaqueous works must be kept in excess of the pressure due to the superincumbent water; that is, it must be increased by one atmosphere, or 15 lb per sq. in. for every 33½ ft. that the caisson is submerged below the surface. Hence at a depth of 100 ft. a worker in a caisson, or a diver in a diving-dress, must be subjected to a pressure of four atmospheres or 60 lb per sq. in. Exposure to such pressures is apt to be followed by disagreeable and even dangerous physiological effects, which are commonly referred to as caisson disease or compressed air illness. The symptoms are of a very varied character, including pains in the muscles and joints (the "bends"), deafness, embarrassed breathing, vomiting, paralysis ("divers' palsy"), fainting and sometimes even sudden death. At the St Louis bridge, where a pressure was employed equal to 4¼ atmospheres, out of 600 workmen, 119 were affected and 14 died. At one time the symptoms were attributed to congestion produced by the mechanical effects of the pressure on the internal organs of the body, but this explanation is seen to be untenable when it is remembered that the pressure is immediately transmitted by the fluids of the body equally to all parts. They do not appear during the time that the pressure is being raised nor so long as it is continued, but only after it has been removed; and the view now generally accepted is that they are due to the rapid effervescence of the gases which are absorbed in the body-fluids during exposure to pressure. Experiment has proved that in animals exposed to compressed air nitrogen is dissolved in the fluids in accordance with Dalton's law, to the extent of roughly 1% for each atmosphere of pressure, and also that when the pressure is suddenly relieved the gas is liberated in bubbles within the body. It is these bubbles that do the mischief. Set free in the spinal cord, for instance, they may give rise to partial paralysis, in the labyrinth of the ear to auditory vertigo, or in the heart to stoppage of the circulation; on the other hand, they may be liberated in positions where they do no harm. But if the pressure is relieved gradually they are not formed, because the gas comes out of solution slowly and is got rid of by the heart and lungs. Paul Bert exposed 24 dogs to pressure of 7-9½ atmospheres and "decompressed" them rapidly in 1-4 minutes. The result was that 21 died, while only one showed no symptoms. In one of his cases, in which the apparatus burst while at a pressure of 9½ atmospheres, death was instantaneous and the body was enormously distended, with the right heart full of gas.
But he also found that dogs exposed, for moderate periods, to similar pressures suffered no ill effects provided that the pressure was relieved gradually, in 1-1½ hours; and his results have been confirmed by subsequent investigators. To prevent caisson disease, therefore, the decompression should be slow; Leonard Hill suggests it should be at a rate of not less than 20 minutes for each atmosphere of pressure. Good ventilation of the caisson is also of great importance (though experiment does not entirely confirm the view that the presence of carbonic acid to an amount exceeding 1 or 1¼ parts per thousand exercises a specific influence on the production of compressed air illness), and long shifts should be avoided, because by fatigue the circulatory and respiratory organs are rendered less able to eliminate the absorbed gas. Another reason against long shifts, especially at high pressures, is that a high partial pressure of oxygen acts as a general protoplasmic poison. This circumstance also sets a limit to the pressures that can possibly be used in caissons and therefore to the depths at which they can be worked, though there is reason to think that the maximum pressure (4¾ atmospheres) so far used in caisson work might be considerably exceeded with safety, provided that proper precautions were observed in regard to slow decompression, the physique of the workmen, and the hours of labour. As to the remedy for the symptoms after they have appeared, satisfactory results have been obtained by replacing the sufferers in a compressed air chamber ("recompression"), when the gas is again dissolved by the body fluids, and then slowly "decompressing" them.
See Paul Bert, La Pression barométrique (1878); and Leonard Hill, Recent Advances in Physiology and Biochemistry (1906), (both these works contain bibliographies); also a lecture by Leonard Hill delivered at the Royal Institution of Great Britain on the 25th of May 1906; "Diving and Caisson Disease," a summary of recent investigations, by Surgeon Howard Mummery, British Medical Journal, June 27th, 1908; Diseases of Occupation, by T. Oliver (1908); Diseases of Workmen, by T. Luson and R. Hyde (1908).
CAITHNESS, a county occupying the extreme north-east of Scotland, bounded W. and S. by Sutherlandshire, E. by the North Sea, and N. by the Pentland Firth. Its area is 446,017 acres, or nearly 697 sq. m. The surface generally is flat and tame, consisting for the most part of barren moors, almost destitute of trees. It presents a gradual slope from the north and east up to the heights in the south and west, where the chief mountains are Morven (2313 ft.), Scaraben (2054 ft.) and Maiden Pap (1587 ft.). The principal rivers are the Thurso ("Thor's River"), which, rising in Cnoc Crom Uillt (1199 ft.) near the Sutherlandshire border, pursues a winding course till it reaches the sea in Thurso Bay; the Forss, which, emerging from Loch Shurrery, follows a generally northward direction and enters the sea at Crosskirk, a fine cascade about a mile from its mouth giving the river its name (fors, Scandinavian, "waterfall;" in English the form is force); and Wick Water, which, draining Loch Watten, flows into the sea at Wick. There are many other smaller streams well stocked with fish. Indeed, the county offers fine sport for rod and gun. The lochs are numerous, the largest being Loch Watten, 2¾ m. by ¾ m., and Loch Calder, 2¼ by 1 m., and Lochs Colam, Hempriggs, Heilen, Ruard, Scarmclate, St John's, Toftingale and Wester. So much of the land is low-lying and boggy that there are no glens, except in the mountainous south-west, although towards the centre of the county are Strathmore and Strathbeg (the great and little valleys). Most of the coast-line is precipitous and inhospitable, particularly at the headlands of the Ord, Noss, Skirsa, Duncansbay, St John's Point, Dunnet Head (346 ft.), the most northerly point of Scotland, Holburn and Brims Ness. From Berriedale at frequent intervals round the coast occur superb "stacks," or detached pillars of red sandstone, which add much to the grandeur of the cliff scenery.
Caithness is separated from the Orkneys by the Pentland Firth, a strait about 14 miles long and from 6 to 8 miles broad. Owing to the rush of the tide, navigation is difficult, and, in rough weather, dangerous. The tidal wave races at a speed which varies from 6 to 12 m. an hour. At the meeting of the western and eastern currents the waves at times rise into the air like a waterspout, but the current does not always nor everywhere flow at a uniform rate, being broken up at places into eddies as perilous as itself. The breakers caused by the sunken reefs off Duncansbay Head create the Bores of Duncansbay, and eddies off St John's Point are the origin of the Merry Men of Mey, while off the island of Stroma occurs the whirlpool of the Swalchie, and off the Orcadian Swona is the vortex of the Wells of Swona. Nevertheless, as the most direct road from Scandinavian ports to the Atlantic the Firth is used by at least 5000 vessels every year. In the eastern entrance to the Firth lies the group of islands known as the Pentland Skerries. They are four in number—Muckle Skerry, Little Skerry, Clettack Skerry and Louther Skerry—and the nearest is 4½ m. from the mainland. On Muckle Skerry, the largest (½ m. by ⅓ m.), stands a lighthouse with twin towers, 100 ft. apart. The island of Stroma, 1½ m. from the mainland (pop. 375), belongs to Caithness and is situated in the parish of Canisbay. It is 2¼ m. long by 1¼ m. broad. In 1862 a remarkable tide climbed the cliffs (200 ft.) and swept across the island.
Geology.—Along the western margin of the county from Reay on the north coast to the Scaraben Hills there is a narrow belt of country which is occupied by metamorphic rocks of the types found in the east of Sutherland. They consist chiefly of granulitic quartzose schists and felspathic gneisses, permeated in places by strings and veins of pegmatite. On the Scaraben Hills there is a prominent development of quartz-schists the age of which is still uncertain. These rocks are traversed by a mass of granite sometimes foliated, trending north and south, which is traceable from Reay southwards by Aultnabreac station to Kinbrace and Strath Helmsdale in Sutherland. Excellent sections of this rock, showing segregation veins, are exposed in the railway cuttings between Aultnabreac and Forsinard. A rock of special interest described by Professor Judd occurs on Achvarasdale Moor, near Loch Scye, and hence named Scyelite. It forms a small isolated boss, its relations to the surrounding rocks not being apparent. Under the microscope, the rock consists of biotite, hornblende, serpentinous pseudo-morphs after olivine and possibly after enstatite and magnetite, and may be described as a mica-hornblende-picrite. The remainder of the county is occupied by strata of Old Red Sandstone age, the greater portion being grouped with the Middle or Orcadian division of that system, and a small area on the promontory of Dunnet Head being provisionally placed in the upper division. By means of the fossil fishes, Dr Traquair has arranged the Caithness flagstone series in three groups, the Achanarras beds at the base, the Thurso flagstones in the middle, and the John o' Groats beds at the top. In the extreme south of the county certain minor subdivisions appear which probably underlie the lowest fossiliferous beds containing the Achanarras fauna. These comprise (1) the coarse basement conglomerate, (2) dull chocolate-red sandstones, shales and clays around Braemore in the Berriedale Water, (3) the brecciated conglomerate largely composed of granite detritus seen at Badbea, (4) red sandstones, shales and conglomeratic bands found in the Berriedale Water and further northwards in the direction of Strathmore. Morven, the highest hill in Caithness, is formed of gently inclined sandstones and conglomerates resting on an eroded platform of quartz-schists and quartz-mica-granulites. The flagstones yielding the fishes of the lowest division of the Orcadian series appear on Achanarras Hill about three miles south of Halkirk. The members of the overlying Thurso group have a wide distribution as they extend along the shore on either side of Thurso and spread across the county by Castletown and Halkirk to Sinclairs Bay and Wick. They are thrown into folds which are traversed by faults some of which run in a north and south direction. They consist of dark grey and cream-coloured flagstones, sometimes thick-bedded with grey and blue shales and thin limestones and occasional intercalations of sandstone. In the north-west of the county the members of the Thurso group appear to overlap the Achanarras beds and to rest directly on the platform of crystalline schists. In the extreme north-east there is a passage upwards into the John o' Groats group
with its characteristic fishes, the strata consisting of sandstones, flagstones with thin impure limestones. The rocks of Dunnet Head, which are provisionally classed with the upper Old Red Sandstone, are composed of red and yellow sandstones, marls and mudstones. Hitherto no fossils have been obtained from these beds save some obscure plant-like markings, but they are evidently a continuation southwards of the sandstones of Hoy, which there rest unconformably on the flagstone series of Orkney. This patch of Upper Old Red strata is faulted against the Caithness flagstones to the south. For many years the flagstones have been extensively quarried for pavement purposes, as for instance near Thurso, at Castletown and Achanarras. Two instances of volcanic necks occur in Caithness, one piercing the red sandstones at the Ness of Duncansbay and the other the sandstones of Dunnet Head north of Brough. They point to volcanic activity subsequent to the deposition of the John o' Groats beds and of the Dunnet sandstones. The materials filling these vents consist of agglomerate charged with blocks of diabase, sandstone, flagstone and limestone.
An interesting feature connected with the geology of Caithness is the deposit of shelly boulder clay which is distributed over the low ground, being deepest in the valleys and in the cliffs surrounding the bays on the east coast. Apart from the shell fragments, many of which are striated, the deposit contains blocks foreign to the county, as for instance chalk and chalk-flints, fragments of Jurassic rocks with fossils and pieces of jet. The transport of local boulders shows that the ice must have moved from the south-east towards the north-west, which coincides with the direction indicated by the striae. The Jurassic blocks may have been derived from the strip of rocks of that age on the east coast of Sutherland. The shell fragments, many of which are striated, include arctic, boreal and southern forms, only a small number being characteristic of the littoral zone.
Climate and Agriculture.—The climate is variable, and though the winter storms fall with great severity on the coast, yet owing to proximity to a vast expanse of sea the cold is not intense and snow seldom lies many days continuously. In winter and spring the northern shore is subject to frequent and disastrous gales from the N. and N.W. Only about two-fifths of the arable land is good. In spite of this and the cold, wet and windy climate, progressive landlords and tenants keep a considerable part of the acreage of large farms successfully tilled. In 1824 James Traill of Ratter, near Dunnet, recognizing that it was impossible to expect tenants to reclaim and improve the land on a system of short leases, advocated large holdings on long terms, so that farmers might enjoy a substantial return on their capital and labour. Thanks to this policy and the farmers' skill and enterprise, the county has acquired a remarkable reputation for its produce; notably oats and barley, turnips, potatoes and beans. Sheep—chiefly Leicester and Cheviots—of which the wool is in especial request in consequence of its fine quality, cattle, horses and pigs are raised for southern markets.
Other Industries.—The great source of profit to the inhabitants is to be found in the fisheries of cod, ling, lobster and herring. The last is the most important, beginning about the end of July and lasting for six weeks, the centre of operations being at Wick. Besides those more immediately engaged in manning the boats, the fisheries give employment to a large number of coopers, curers, packers and helpers. The salmon fisheries on the coast and at the mouths of rivers are let at high prices. The Thurso is one of the best salmon streams in the north. The flagstone quarries, mostly situated in the Thurso, Olrig and Halkirk districts, are another important source of revenue. Of manufactures there is little beyond tweeds, ropes, agricultural implements and whisky, and the principal imports consist of coal, wood, manure, flour and lime.